In the anime

Honchkrow's wings become outlined in light blue. The outline disappears as it releases multiple light blue crescents by flapping its wings. Its wings then become outlined again as the crescents go for the opponent.

Trivia

The Bag sprite for the HM containing Defog is mistakenly that of a Water-type HM, rather than a Flying-type one.

Like Flash was in Generations I, II, and III, Defog is not critical to the completion of the game, as a player can traverse foggy areas without using the move. However, unlike with dark caves requiring Flash to light them, foggy areas affect battles within them.

If a player teaches a Pokémon Defog by HM in Diamond, Pearl, and Platinum and trades it to HeartGold or SoulSilver, the Pokémon can be freely imported to Black and White via the Poké Transfer, due to Defog not being needed outside of battle in HeartGold and SoulSilver. The same can be done with Whirlpool by transferring a Pokémon that learned it by HM in HeartGold or SoulSilver to Diamond, Pearl, or Platinum.